proposal: help and completion frameworks

[kyleburton]

# @help.cmd:cmd|oneline help text
# @help.long:cmd|multi line
# @help.long:cmd|help descriptions

# @help.cmd:cmd:subcmd|oneline help text
# @help.long:cmd:subcmd|multi line
# @help.long:cmd:subcmd|help descriptions

# @completions:<<cmd>>:opts|<<options for --foo or --bar=qux style parameters>> i.e. not counted as positional
# @completions:<<cmd>>:2|local dev stage preprod prod
# @completions:<<cmd>>:3|<<options for position 3>
# @completions:<<cmd>>:4|<<options for position 4>   
# @completions:help:2!<<shell-function-to-gen-opts-for-position-2>                                                                        

# how might we support completion for sub-commands and options specific to sub commands?